A magnetic rotary encoder system has been known to be a highly useful instrument to measure an angular or linear movement of a movable member such as in a machine tool. Indeed, precision measurement of such a movement can be basis for accurately positioning a movable part, e.g. a tool or workpiece, in machine and other tools.
In these systems of conventional type, magnetic disks have a common scaling configuration in which the successive magnetic scaling units are arranged along a circle on one surface of the disk coaxial therewith so that the opposed magnetic poles constituting each of the scaling units extends in alignment with the rotary direction of the disk. The magnetic sensing head is disposed in spaced juxtaposition with that one surface such as to succcssively sense these scaling units which are swept along that circle as the disk is rotated. It is recognized that this magnetic disk structure is characterized by a relatively poor density of magnetic encoding or scaling units per area (angle) and hence an unsatisfactory resolving power for magnetic sensing.
Rotary magnetic disks of the other known type make use of a scale arrangement in which the disk is magnetized across the thickness thereof so that magnetic scaling units, each of which extends perpendicularly to the side surfaces of the disk, are successively formed in a mutually parallel relationship along a circular zone coaxial with the disk. The magnetic sensing head is then C- or U-shaped to place between its two sensing arms the rotating disk. Such a disk must have a considerable thickness to present magnetic fluxes of a satisfactory strength from each of the scaling units. Thickening the disk entails an increase in its weight and it has been found that this may impose an undue load on the sensing movement of a member moving to rotate the disk.
In both types of conventional magnetic encoder systems described, it should also be noted that a thin rotating disk tends to undulate and the undulating movement of the rotating disk is accentuated towards the periphery thereof adjacent to which the magnetic scale units are arranged. As a result, the rotating disk and the sensing head which are spaced apart across a small air gap may come into a sliding contact, causing an undesirable wear of the head surface. Furthermore, a considerable change in the magnetic gap spacing is created and tends to result in a sensing error.